Transmissions typically control gear ratio changes in accordance with a shift map or based on gear selections by an operator. Specifically for work machines, such as tractors and motor graders, transmission may incorporate automatic shifts to maintain a desired groundspeed. However, when the machine is working, the operators will typically disable the automatic shifting to ensure that the machine will have sufficient torque to, for example, overcome obstacles such as boulders and the like, and to travel on inclines, and the like.
Power trains that include transmissions connected to engines through torque converters, and/or other components, can often operate at different gear ratios, engine speeds, and torque converter settings to provide a substantially constant ground speed for the machine, which is selected by an operator. Any one of several different combinations of engine speed and transmission gear ratio may provide a desired groundspeed. Because of this inherent flexibility, although an operator may select an appropriate gear ratio to perform a machine function, such selection may not always be the most efficient from the operating standpoint of the machine, which can result in increased noise and reduced fuel efficiency.
One example of an automatically shifting transmission can be seen in U.S. Pat. No. 6,496,767 (the '767 patent), which issued on Dec. 17, 2002. The '767 patent describes a method for determining shift points in a step gear transmission system that maximizes fuel economy. The method uses fuel economy data from various gear ratios to calculate decision curves. These decision curves are then used during operation to minimize fuel consumption while maintaining a desired ground speed.
Although the method disclosed in the '767 patent is effective in reducing fuel consumption in machines, it cannot assure that the best overall operating efficiency of a machine can be achieved under a broad range of operating conditions. Moreover, it cannot prevent operation in an inefficient gear that is selected by an operator that overrides the automatic shifting functionality.